I definately feel that a polite sound is due to a slight lack of top end extension. If you are missing the high frequencies, the leading edges get rounded, dynamics are softer, the sound doesn't jump out at you, the bass becomes more noticible, diction becomes a little harder to understand, contrasts aren't as clear, speed slows down, etc.
However, there is a postive side (remember that with any positive there is a negative, and vice versa. Everything in nature is a compromise). The sound will not be harsh or bright, poor recordings are more enjoyable, a digital-sounding CD player will be nicer, loud volumes will be impressive rather than painful, classical music takes on a more emotional role, the interconnections between notes will be more obvious, flow of music will be analog, etc.
IMO and IME, the politeness of my Classe amps has been welcome when combined with other components that are more mouthy and pushy. There always has to be a balance so I don't feel that anyone can criticize politeness or forwardness unless the entire system is one or the other.
Some people prefer forward gear and then they will say that (forward) CD sounds like crap and you need a (polite) turntable to make it all better. I prefer the opposite: a more polite amp with a CD player. They are simply two different ways at solving the same problem. Either way, you have to find the right balance. There is no right or wrong.
Arthur